This invention relates to a device for automatically controlling the level of an analog signal.
A device for automatically controlling the level of an analog signal is broadly classified into two types: dynamic level compressor (DLC) and dynamic level expander (DLE). Both types are included in an automatic level control device in the broad sense. These types DLC, DLE are widely used with a noise reduction system, etc. Particularly, the DLC type is often used with a tape recorder. The DLC for a tape recorder is generally referred to as an automatic level controller (ALC), and is used to record sounds with a proper signal level, regardless of the magnitude of the level of a recording input signal.
FIG. 1 shows the basic arrangement of the above-mentioned automatic level-controlling device. An input signal Ein is supplied from an input terminal A to a variable gain circuit 10. At this time, a first signal E10 corresponding to the input signal Ein is delivered from the variable gain circuit 10 to a detector 20, which in turn sends forth a second signal E20 having a DC level corresponding to the magnitude of the first signal E10. The second signal E20 is supplied as a control signal to a variable impedance element 30, which is connected to the variable gain circuit 10 through a connection line LC. Where the arrangement of FIG. 1 constitutes the aforementioned automatic level controller ALC, then the average level of an output signal Eout from the variable gain circuit 10 is restricted within a certain prescribed range regardless of the magnitude of the level of an input signal Ein. An output signal Eout whose level has thus been automatically controlled is issued from an output terminal B.
Hitherto, the detector 20 is often formed of a diode or a circuit utilizing a rectifying action taking place across the base-emitter region of a transistor. The detector 20 has to produce a second signal E20 having a higher level than the input threshold level of the variable impedance element 30. The reason is that where the second signal E20 has a lower level than the input threshold level, then the automatic control of a signal level can not be effected. Therefore, the first signal E10 which corresponds to the second signal E20 having a higher level than said input threshold level has to be supplied to the detector 20. With the conventional voltage arrangement, it has been necessary to let the first signal E10 or output signal Eout have a sufficiently high level in order to meet the above-mentioned requirement. This requirement has rendered the conventional circuit unadapted to be used with a device operated at a low voltage level of the order of 2 to 3 volts. The reason why a low signal level and a low power supply voltage are not suitable for the conventional circuit is discussed in the U.S. patent application "Automatic Level Control Device", U.S. Ser. No. 275,347, filed on June 19, 1981, inventor: Kenji FUJIBAYASHI. The disclosure of the above U.S. application is now combined herewith.